Zealandia: Our Continent Revealed

When James Cook rounded Cape Horn and entered the Pacific aboard Endeavour in January 1769, his first job was a scientific one—to observe the transit of Venus. But Cook was also under Admiralty instructions to search for a long-suspected southern continent—Terra Australis Incognita. In 1770, having spent six months charting the coast of New Zealand, Cook sailed west, convinced that it was not part of the continent he was looking for.

Geologists Nick Mortimer and Hamish Campbell think he was mistaken, and they have written Zealandia: Our Continent Revealed to prove it. The great explorer failed, they say, because Terra Australis Incognita lay under Endeavour’s hull.

“Imagine a typical continent with seem­ingly endless land in all directions. There are broad valleys and uplands, wide-open vistas across undulating plains and upstanding mountain ranges far in the distance… And there may be canyons, valleys, gorges, large depressions and basins. Now imagine this same continent under the sea, and largely drowned.”

That, say Mortimer and Campbell, is Zealandia. The landmass of New Zealand straddles its centre and, away to the north, New Caledonia marks its visible tropical extreme.

But, unlike Earth’s other six conti­nents, most of Zealandia—95 per cent to be precise—lies beneath the water. A great deal of investigative work, using an array of scientific tools and centuries of accumulated knowledge, has been required to understand and map it.

The most fundamental prerequisite, say the authors, is to understand—in terms of geology as opposed to geography—what makes a continent a continent. This hinges on its thickness, the rocks involved and, where it is below sea level, the depth at which it lies. Oceanic crust is largely composed of basalt, produced by undersea volcanoes. The thicker continental crust, by contrast, is more varied and includes granite, rhyolite, schist and greywacke. These rocks are less dense than basalt, and float higher on the soft part of the Earth’s mantle (the asthenosphere) than oceanic crust does.

Marine geology and geophysics have been key in outlining the extent of Zealandia. Scientists have retrieved continental rocks far out at sea, meas­ured the crust’s thickness and used bathymetry to trace where the edge of the continental slope meets the abyssal plain. A great advantage of this geological approach to mapping Zealandia is that its size and shape are not affected by changing sea level. The extent of Zealandia—and for that matter, of Africa, Australia and the rest—remains constant, come ice age or high water.

Nevertheless, Mortimer and Campbell vividly picture how the newly discovered continent would have looked to Kupe and Cook if the waters of the Pacific had obligingly dropped to expose its full extent. It measures 4000 kilometres in length and is 17 times the area of present-day New Zealand. When mapped, it resembles a tilted and much enlarged Britain, with New Zealand as a mountain chain running through its waist and New Caledonia—serendipi­tously, given its name­ overlaying Scotland. A river system drains southeast beneath what the authors label the Chatham Peninsula, and another river, longer than the Colorado, runs north.

Surprisingly, Charles Hursthouse, an early colonist, foreshadowed all this in his 1857 book New Zealand or Zealandia, the Britain of the South. Hursthouse was no scientist. He wrote before systematic geological surveys had begun, and largely to advance the cause of settlement. Yet he penned these astonishing words: “The geology of New Zealand adopts the theory that New Zealand is but a part of a great continent which has been submerged…”.

A century and a half later, a United Nations commission accepted New Zealand’s claim to an extended 5.857-million-square­kilometre continental shelf. By then, the name Zealandia was increasingly being used—after the example of American geosci­entist Bruce Luyendyk in 1995—to describe the continental crust on which New Zealand sits.

All well and good, but why does any of this matter? Because, say Mortimer and Campbell, the shift from “islands” to “conti­nent” has massive implications in terms of both natural resources and national identity.

“Whether we care to admit it or not, what makes or breaks nations and allows them to celebrate, preserve and promote their culture and environment instead of simply surviving off them is largely to do with access to resources… Zealandia is of continental proportions and hence its potential energy and mineral resources are of continental proportions.”

Perhaps, then, it is time for us to get better acquainted not just with the Chatham Rise, but with the vast expanses of the Campbell Plateau, the Lord Howe Rise and the Challenger Plateau. All we lack to profit from them are environmentally acceptable techniques and extraction technologies we can afford.